Hidden caretaker

In a fantasy world where all the humanoid species live together peacefully, unified against other "lesser" mythical creatures, a mythical creature of your choice has found itself living in the storm drainage system under a sprawling metropolis. They have lived there fine for their entire life but after finding the young of another mythical species with no guardian in sight and adopting it they must figure out how to take care of it while keeping both of them safe from the people above